One prior art composite magnetic material used as the magnetic material for multilayer electronic parts is ferrite powder having a mean particle size of several hundreds of nanometers to several tens of micrometers mixed and dispersed in an organic material (see Japanese Patent Application No. 9-76341). The composite magnetic material is applied to glass cloth to form a prepreg, and a copper foil is clad to the prepreg to yield a copper-clad laminate. By forming a desired pattern on this laminate, an inductance device having improved high-frequency characteristics is obtained.
Known materials for multilayer substrates or magnetic substrates using prepreg include magnetic metal particles mixed and dispersed in resins (see JP-A 8-78798 and JP-A 10-79593). Also JP-A 8-204486 discloses a composite magnetic material having spherical iron carbonyl dispersed in a resin.
A molding material using a composite magnetic material is disclosed in JP-A 7-235410 as comprising spherical iron particles having a mean particle size of about 50 μm which are surface insulated and bound in a resin, the material being used as the core of motors and transformers.
An electromagnetic shield material is described in Journal of Applied Magnetic Society, Vol. 22, No. 4-2, 1998, pp. 885-888, as comprising magnetic metal particles of a small size. The material keeps a higher complex permeability even to higher frequencies than cubic ferrite and is expected to have a shielding effect even at higher frequencies.
However, composite magnetic materials comprising magnetic metal particles except for ferrite and resins are low in insulation and have a greater imaginary part of complex permeability (the imaginary part becomes greater with higher electroconductivity) which impedes attenuation. An attempt was made to treat magnetic metal particles with a coupling agent for enhancing insulation.
Of prior art materials for use as molding materials for high-frequency electronic parts (molding materials adapted for transfer molding and injection molding), casting materials (liquid materials adapted for casting by potting), paints such as printing paste, powder compression molding materials (material to be molded by compression), prepreg and substrates, one example is illustrated in FIG. 87A as a composite dielectric material 232 comprising a powder of dielectric particles 230 adjusted to a particle size of several hundreds of nanometers to several tens of micrometers dispersed in a resin 231. When this composite dielectric material 232 is used in a laminate substrate, for example, as shown in FIG. 87B, the composite dielectric material 232 is applied to a glass cloth 233 for impregnation, forming a prepreg 234 as an intermediate product to the laminate substrate. Copper foils are clad to the prepreg to form a laminate, on which a desired conductor pattern is formed by a printed circuit board manufacturing process. The dielectric powder used in this composite dielectric material is obtained by firing a powder or grinding a sintered dielectric material. The properties of the sintered dielectric used herein are selected by taking into account a dielectric constant and tan δ since they are closely correlated to the properties of the finally finished composite dielectric material.
An electronic part such as a capacitor or piezoelectric device is constructed as shown in FIG. 87C by bonding external electrodes 235 to opposite surfaces of the composite dielectric material 232.
Regarding Glass Cloth-loaded Prepreg and Copper-clad Magnetic Substrate
(a) When an inductance device is fabricated using a composite magnetic copper-clad substrate comprising a ferrite powder mixed and dispersed in an organic material, the use of a high permeability ferrite powder tends to exacerbate high-frequency characteristics, and inversely, the use of a low permeability ferrite powder ensures good high-frequency characteristics, but fails, of course, to provide a sufficient permeability. Satisfactory characteristics are not available in either case.
(b) When a metal magnetic material, for example, iron carbonyl is used instead of the ferrite powder, there is obtained a composite magnetic substrate having a relatively high permeability and good high-frequency characteristics, but poor insulation. The poor insulation of a fired body (composite magnetic material) gives rise to the inconvenient problem in the copper foil patterning step that a plating metal can deposit on areas outside patterned sections, causing short-circuits between patterned sections. When silicon iron is used instead, there is obtained a composite magnetic substrate which has a high permeability and saturation magnetic flux density, but suffers from problems including prohibited use in the high-frequency region and poor insulation.
Regarding Magnetic Molding Material
(1) Sheet Forming Material
(a) A shield member, which is obtained by mixing and dispersing a soft magnetic metal powder such as iron carbonyl or silicon iron in a resin and molding the mixture into a sheet, can have a volume resistivity of 107 Ω-cm if the metal powder is subjected to coupling treatment or oxidation on the surface. However, the withstanding voltage is only about 150 V at a thickness of 1.0 mm. Then the member is not regarded as electrically insulating upon application of a voltage, with a potential of electrical short-circuiting.
(b) A shield member having ferrite powder dispersed therein instead of the soft magnetic metal powder such as iron carbonyl or silicon iron has a high volume resistivity and substantially eliminates the potential of electrical short-circuiting. However, it is not only ineffective for electric field shielding, but also less effective for magnetic shielding on the low-frequency side.
(2) Molding Material
As a countermeasure for radiant noise of a printed circuit board having parts mounted thereon, it is customary to mold a molding material over the part-mounting surface so that the composite magnetic material having ferrite mixed with a resin may cover the parts entirely.
The molding material having ferrite powder dispersed therein is not only ineffective for electric field shielding, but also less effective for magnetic shielding on the low-frequency side. The molding material having a soft magnetic metal powder such as iron carbonyl or silicon iron dispersed therein has an increased shielding effect, but is less insulating, inviting performance defects due to the poor insulation between patterns on the circuit board.
(3) Composite Magnetic Core Material
Composite magnetic materials for use as the core of choke coils and transformers include ferrite particles having a mean particle size of several hundreds of nanometers to several tens of micrometers or magnetic metal particles surface treated for insulation dispersed in resinous materials such as liquid crystal polymers, PPS resins and epoxy resins. The materials are molded into the desired shape to serve as a magnetic core. The core having ferrite dispersed therein is difficult to use in a high current, high power application because its saturation magnetic flux density is low. The core using magnetic metal material provides insufficient insulation, leaving a reliability problem.
Regarding Magnetic Paint
For the application to form a magnetic circuit in a reactance device or to paint to form a magnetic shield, it is customary to form a composite magnetic film by mixing and dispersing ferrite in a resin and solvent to form a printing paste and applying the paste by screen printing. The composite magnetic material using ferrite fails to provide a sufficient permeability and saturation magnetic flux density and encounters difficulty on practical use. The use of magnetic metal powder instead of ferrite is also customary, but provides insufficient insulation, resulting in reactance devices having poor properties and a failure by electrical short-circuit with the surrounding metal at the shield surface.
Regarding Magnetic Powder Compression Molding Material
For preparing a composite magnetic material with high loading of magnetic metal powder, it is customary to mix the magnetic powder with several weight percents of a resin such as an epoxy resin and mold the mixture under heat and pressure. However, since sufficient insulation is not insured, the metal surface is oxidized or otherwise treated. Nevertheless, such treatment is not satisfactory. The withstanding voltage is below the practical level. Additionally, the oxide film is so weak that if the molding method entails application of a high pressure upon molding, it can be broken under the applied pressure. It is thus difficult to derive the full advantages of the oxide film.
Regarding Prior Art Composite Dielectric Material
In the case of the electronic part constructed of a prior art composite dielectric material as illustrated in FIG. 87C, dielectric particles 230 of a distinct material exist dispersed in the resin 231 between the external electrodes 235 and 235. Here the resultant dielectric constant is determined by a volume ratio of these two materials.
FIG. 88A illustrates measurements of resultant dielectric constant (ε) of materials in which a dielectric powder having a dielectric constant (ε) of 9,000 or a dielectric powder having a dielectric constant (ε) of 90 is dispersed in an epoxy resin in varying amounts.
As seen from FIG. 88A, the material having 60 vol % of the dielectric powder having a dielectric constant of 9,000 dispersed in the epoxy resin has a resultant dielectric constant of about 40, which is reduced to about 1/200 of the dielectric powder's dielectric constant, indicating that mixing a dielectric material having a high dielectric constant does not provide a so high dielectric constant. In this regard, for the material having the powder with a dielectric constant of 90 dispersed in the epoxy resin, the resultant dielectric constant at a power content of 60 vol % is about 20, which is reduced to about ⅕. Also, the material having 40 vol % of the powder having a dielectric constant of 9,000 dispersed in the epoxy resin has a resultant dielectric constant of about 15, and the material having 40 vol % of the dielectric powder having a dielectric constant of 90 dispersed in the epoxy resin has a resultant dielectric constant of about 12, with no significant difference being ascertained therebetween.
By diluting the composite dielectric materials with a solvent, and impregnating glass cloths therewith, double sided copper-clad substrates were prepared for examining the relationship of dielectric constant to the powder content, with the results shown in FIG. 88B. As seen from FIG. 88B, when the glass cloth is impregnated with the composite dielectric material, the difference in dielectric constant of the dispersed powder does not manifest in the composite dielectric material as in the absence of glass cloth. This is because the volume fraction for which glass cloth accounts in the substrate becomes non-negligible so that the glass cloth having a dielectric constant of 7.0 has an influence on the resultant dielectric constant which is otherwise determined by the volume fraction.
As seen from the above, at least 60 vol % of the powder having a dielectric constant of 9,000 must be dispersed in order to provide conventional composite dielectric materials with a high dielectric constant. However, in order to form a thin substrate, the content of the composite dielectric material must be 50 vol % or less when adhesion with copper foil and delamination are taken into account. Then, even though an expensive dielectric powder is mixed, a significant improvement in dielectric constant is not achievable. Since the dielectric powder is previously obtained by grinding sintered dielectrics, such particles have bosses and recesses and a large particle size and are thus less dispersible, rendering it difficult to stabilize the properties of electronic parts (e.g., thin gage capacitors and piezoelectric devices) and substrates.
Regarding Electronic Part Using Dielectric Material
In the case of electronic parts constructed using conventional composite dielectric materials, dielectric particles of a distinct material exist dispersed in the resin between the external electrodes. Here the resultant dielectric constant is determined by a volume ratio of these two materials.
Mixing a dielectric material having a high dielectric constant does not provide a so high dielectric constant. For instance, for the material having a powder with a dielectric constant of 90 dispersed in an epoxy resin, the resultant dielectric constant at a power content of 60 vol % is about 20, which is reduced to about ⅕ of the original. Also, the material having 40 vol % of a powder having a dielectric constant of 9,000 dispersed in an epoxy resin has a resultant dielectric constant of about 15, and the material having 40 vol % of a powder having a dielectric constant of 90 dispersed in an epoxy resin has a resultant dielectric constant of about 12, with no significant difference being ascertained therebetween.
When glass cloth is impregnated with a composite dielectric material, the difference in dielectric constant of the dispersed powder does not manifest in the composite dielectric material as in the absence of glass cloth. This is because the volume fraction for which glass cloth accounts in the substrate becomes non-negligible so that the glass cloth having a dielectric constant of 7.0 has an influence on the resultant dielectric constant which is otherwise determined by the volume fraction.
As seen from the above, at least 60 vol % of the powder having a dielectric constant of 9,000 must be dispersed in order to provide conventional composite dielectric materials with a high dielectric constant. However, in order to form a thin substrate, the content of the composite dielectric material must be 50 vol % or less when adhesion with copper foil and delamination are taken into account. Then, even though an expensive dielectric powder is mixed, a significant improvement in dielectric constant is not achievable. Since the dielectric powder is previously obtained by grinding sintered dielectrics, such particles have bosses and recesses and a large particle size and are thus less dispersible, rendering it difficult to stabilize the properties of electronic parts (e.g., thin gage capacitors and piezoelectric devices) and substrates.
Regarding Electronic Part Using Glass Cloth-loaded Prepreg and Copper-clad Magnetic Substrate
(a) When an inductance device is fabricated using a composite magnetic copper-clad substrate comprising a ferrite powder mixed and dispersed in an organic material, the use of a high permeability ferrite powder tends to exacerbate high-frequency characteristics, and inversely, the use of a low permeability ferrite powder ensures good high-frequency characteristics, but fails, of course, to provide a sufficient permeability. Satisfactory characteristics are not available in either case.
(b) When a metal magnetic material, for example, iron carbonyl is used instead of the ferrite powder, there is obtained a composite magnetic substrate having a relatively high permeability and good high-frequency characteristics, but poor insulation. The poor insulation of a fired body (composite magnetic material) gives rise to the inconvenient problem in the copper foil patterning step that a plating metal can deposit on areas outside patterned sections, causing short-circuits between patterned sections. When silicon iron is used instead, there is obtained a composite magnetic substrate which has a high permeability and saturation magnetic flux density, but suffers from problems including prohibited use in the high-frequency region and poor insulation.
Regarding Electronic Part Using Magnetic Molding Material
(1) Sheet Forming Material
(a) A shield member, which is obtained by mixing and dispersing a soft magnetic metal powder such as iron carbonyl or silicon iron in a resin and molding the mixture into a sheet, can have a volume resistivity of 107 Ω-cm if the metal powder is subjected to coupling treatment or oxidation on the surface. However, the withstanding voltage is only about 150 V at a thickness of 1.0 mm. Then the member is not regarded as electrically insulating upon application of a voltage, with a potential of electrical short-circuiting.
(b) A shield member having ferrite powder dispersed therein instead of the soft magnetic metal powder such as iron carbonyl or silicon iron has a high volume resistivity and substantially eliminates the potential of electrical short-circuiting. However, it is not only ineffective for electric field shielding, but also less effective for magnetic shielding on the low-frequency side.
(2) Molding Material
As a countermeasure for radiant noise of a printed circuit board having parts mounted thereon, it is customary to mold a molding material over the part-mounting surface so that the composite magnetic material having ferrite mixed with a resin may cover the parts entirely. The molding material having ferrite powder dispersed therein is not only ineffective for electric field shielding, but also less effective for magnetic shielding on the low-frequency side. The molding material having a soft magnetic metal powder such as iron carbonyl or silicon iron dispersed therein has an increased shielding effect, but is less insulating, inviting performance defects due to the poor insulation between patterns on the circuit board.
(3) Composite Magnetic Core Material
Composite magnetic materials for use as the core of choke coils and transformers include ferrite particles having a mean particle size of several hundreds of nanometers to several tens of micrometers or magnetic metal particles surface treated for insulation dispersed in resinous materials such as liquid crystal polymers, PPS resins and epoxy resins. The materials are molded into the desired shape to serve as a magnetic core. The core having ferrite dispersed therein is difficult to use in a high current, high power application because its saturation magnetic flux density is low. The core using magnetic metal material provides insufficient insulation, leaving a reliability problem.
An object of the invention is to provide a composite magnetic material which is highly electrically insulating, easy to work in preparing a molding material having a high saturated magnetic flux density, free of a corrosion problem, and has improved high-frequency characteristics and withstanding voltage as well as a magnetic molding material, magnetic powder compression molding material, magnetic paint, prepreg, and magnetic substrate using the same.
Another object of the invention is to provide a composite dielectric material comprising effectively dispersible particles, which is readily formulated to provide desired characteristics and suitable for the fabrication of thin gage electronic parts as well as a molding material, powder compression molding material, paint such as a printing paste or casting material, prepreg and substrate using the same.
A further object of the invention is to provide a composite dielectric material which exhibits a high dielectric constant even at a low content of dielectric and requires a low material cost as well as a molding material, powder compression molding material, paint such as a printing paste or casting material, prepreg and substrate using the same.
A yet further object of the invention is to provide an electronic part which is constructed by a material comprising effectively dispersible particles so that desired characteristics and a size reduction are achievable.
A yet further object of the invention is to provide an electronic part which is highly insulating, pressure resistant and free of a corrosion problem, and has improved high-frequency characteristics.